Update,
I have managed to force my iMac to pick up only the 5ghz network. I did this by changing the 2.4ghz password to something that was incorrect, thus forcing the iMac never to connect to the 2.4 side. It seems a bit naff, but so far it works. The Express picks up the 5ghz fine.
One thing I don't understand is that I have read that the 5ghz side should be on a high numbered channel, and when set to auto it sets the 5 at channel 100 And the 2.4 to channel 6. Works so far, but if I need at some point to change the 2.4 channel, it will only allow me to choose a channel for the 5ghz side up to something in the 40's if I remember correctly.
Sorry for the long post.
Neil